[FEATURE]

对于嵌套多层的json字符串能不能不要过于“智能”，一层一层手动解析即可，不要一次性全部解析，导致从fastjson升级过来，项目运行不报错，但是涉及到解析的就报错，还有就是报错信息能不能更加完整一点

No file or test is identified in the issue. First reproduce the fastjson-to-fastjson2 migration with a nested JSON string and clarify the expected one-layer-at-a-time behavior and complete error details; then locate the relevant parsing entry point and add focused coverage, with done defined by the agreed behavior and no regression.

Written by the indexing model from the issue text.